DALY. — THE NATURE OF VOLCANIC ACTION. 



93 



mere mechanical result of explosion. There must be a preliminary 

 weakening of the plug, and apparently the only cause for that weaken- 

 ing is to be found in the fluxing by juvenile gas. 



First we may consider the case where the terrestrial forces keep the 

 liquid column supported in the conduit. With the formation of the 

 plug, the loss of heat falls to a very low rate as compared with that 

 ruling in the active period. Until the plug is removed, nearly all the 



CRATELR 



FioritE 9. Section of upper part of a dormant cone, showing some progress 

 in Ras-fluxing. Broken line iu middle of vent shows original depth of the solid 

 plug. 



loss is due to conduction and is very slow. Two-phase convection is 

 slowed down, but the rise of bubbles does not cease nor does the vol- 

 canic furnace cease working, since a' renewed concentration of juvenile 

 gas is begun. To that positive source of heat in the conduit is to be 

 added the heat developed by the compression of the gas as it accumu- 

 lates beneath the plug and as it is sijueezed by any upthrusting of the 

 magmatic column due to crustal movement. Gradually the lowest part 

 of the plug becomes lic^uefied, preferably along its vertical axis, where 

 the heat inherited from the last active period preserves the line of max- 

 imum temperature in the whole upi)er ])art of the volcano. The relicjue- 

 fied lava sinks into the column, dissolving some of the accumulating 

 gas, so that heat of solution is })robably to be added to the other sup- 

 plies which tend to threaten the existence of the plug. Hence, at least 

 three processes cooperate in fusing the plug ; these are : heat of chemical 

 reaction, heat of gas compression, and heat of gas solution. As the 

 ])lug is thus weakened, the gas-tension increases and activity is renewed 

 by (jne or more major explosions, shattering the remaining part of the 

 plug. (Figure 9.) 



